Optical access systems, such as Passive Optical Networks (PONs) or other fiber optics networks, are continuously growing and optical fibers are being increasingly deployed. The increase in the quantity of deployed fibers can cause complications and communication problems in the systems, and therefore requires reliable diagnostic tools and equipment. One technique that is used to detect physical problems in the fibers is Optical Time Domain Reflectometry (OTDR). In OTDR, a test signal comprising a light pulse is transmitted down the fiber, reflected at some point, and then measured using an OTDR receiver located at the same location where the pulse has originated. The delay time between transmission and reception of the pulse is used to calculate the round-trip distance of the pulse. The calculated round-trip distance is used to find the location of any deteriorations or breaks in the fiber, which may then be fixed or replaced.
Generally, a conventional OTDR device is manually connected to a fiber to be tested, e.g. when a fault occurs. However, in the case of large number of fibers in the system, manually testing the fibers may be labor intensive and prone to many test errors. Alternatively, a single OTDR system may be permanently installed at a central office and shared for a plurality of fibers using moving parts and optical switches, which reduces reliability and increases costs. More recently, embedded OTDR systems have been used to overcome some of the disadvantages of the conventional OTDR system. Specifically, OTDR systems have been embedded into the optical transmitters/receivers (or transceivers) of the communication system. The OTDR signal can then be superimposed on an optical signal using the same transmitter, e.g. as a superimposed sinusoidal signal or specified data pattern. By using the same transmitter, the embedded OTDR systems may save some cost but may also add design complexities to the transmitters and other components. Specifically, the embedded OTDR schemes require specialized receivers to isolate and separately detect the reflected OTDR signals from the transported optical signals in the system. As such, the overall cost in the system may increase. In some schemes, the cost is reduced by using low cost OTDR receivers at the expense of reducing OTDR signal quality and hence testing capability.